Drilling of essentially vertical wellbores into the earth to penetrate subsurface geologic formations containing desirable minerals such as oil, gas, coal, uranium, sulfur, and the like is well known. A large number of minerals bearing formations in the earth are horizontal or essentially horizontal, i.e., within 45.degree. of horizontal, with the earth's surface. A technique has been devised which is well known in the art as "Drainhole Drilling" wherein the vertical wellbore is rapidly turned into a horizontally or essentially horizontally extending wellbore so that the drainhole portion of the wellbore can be extended outwardly within the essentially horizontal formation a substantial distance. This provides for better drainage of the mineral bearing formation since the wellbore rather than just penetrating through the thickness of the formation as would be done with a vertical wellbore, actually extends a substantial distance within the mineral bearing reservoir or formation itself.
A goal when initially completing a drainhole wellbore is to obtain adequate essentially horizontal isolation of the drainhole portion of the wellbore within the productive formation. This goal is desired in order to maximize the useability of the wellbore for future production of minerals and for future remedial operations to be carried out within the wellbore itself so as to enhance the productive life of the well. Currently, in essentially vertical wellbores, the primary method for obtaining vertical isolation of the wellbore in the productive zone of the formation is to run steel casing into the wellbore and inject cement in the annulus which extends between the exterior of the casing and the wellbore wall. This casing and cementing technique allows for individual productive intervals to be perforated, treated, and later squeezed with cement, in needed, without adversely affecting other productive zones along the wellbore length. Put another way, this technique gives vertical zone isolation in vertical wellbores. However, when this technique is applied to drainhole wellbores, adequate horizontal isolation within the productive formation is rarely achieved. This is so because in the drainhole wellbore context gravity is working against the uniform displacement of cement around the annulus outside the casing rather than helping such cement distribution as it does in the vertical wellbore context.
A major problem encountered in employing the casing-cementing completion method in a drainhole wellbore is the very low probability of obtaining a successful cementing job. Because of the effects of gravity, obtaining uniform cement displacement around the casing, which has to be accomplished in order to obtain the desired isolation of productive zones along the drainhole wellbore length, becomes progressively more difficult as the wellbore approaches horizontal. This is because in a drainhole wellbore gravity causes (1) the cement to channel through and not completely displace the drilling mud in the casing annulus due to the different densities of the mud and cement, and (2) the casing strip itself to lay on the lower side of the wellbore thereby decentralizing the casing so that cement cannot uniformly be displaced completely around the casing itself. These effects can lead to incomplete isolation in the casing annulus to an extent that makes it uneconomical in most cases even to attempt to achieve horizontal isolation in a drainhole wellbore with the casing-cementing technique.
As a result, nearly all drainhole wellbores are completed with an uncased, open wellbore in competent productive formations, or some type of slotted liner in unconsolidated formations. However, these methods provide no isolation capability along the horizontal or drainhole portion of the wellbore and this limits greatly the possibility of future remedial operations for the drainhole portion of the wellbore. For example, these methods do not allow for easy identification and isolation of gased out or watered out sections of formation along the drainhole portion of the wellbore.